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Disadvantages of exterior wood furnaces

Mini caddy wood furnaceWe were thrilled when Mark's mom gave us an external wood furnace a couple of years ago, and we are still very grateful.  However, over time, we've discovered that external wood furnaces have a variety of problems associated with them:

  • Extremely inefficient.  Even the most efficient new wood furnance I could find in my web searches (the Mini-Caddy, listed as an Energy Star furnace, and pictured here) is vastly inferior to moderately efficient wood stoves.  You can download a list of EPA certified wood stoves which will tell you the emissions and efficiency of all tested models.  The Caddy, which the Mini-Caddy is based on, has 6.6 g/hr emissions and 63% efficiency.  Compare it, for example, to the Drolet Savannah wood stove (which has slightly less output than the Mini-Caddy and is much cheaper)  --- 6.28 g/hr emissions and 75% efficiency.
  • Expensive.  We got very lucky and were given our furnace as a gift, so I wasn't prepared for the price tag when we considered upgrading to a more efficient model.  The Mini-Caddy seems to retail for somewhere in the $2,000 range, compared to efficient, non-catalytic wood stoves that can be got for closer to $700 (and which also have higher efficiency, don't forget.)  You can get a 30% federal tax credit on either purchase.
  • Smokey wood furnaceNot suited for indoor use.  After finishing up the East Wing, we moved in our wood furnace, figuring that we'd be able to capture some of the heat previously lost to the outside air.  I know that the term "exterior wood furnace" should have tipped me off, but we were shocked at how smoky the East Wing got when we lit our first fire this year.  Granted, our model (LTD Limited by Jordahl Mfg.) is very old, and its drawing problem may not be found in more modern wood furnaces.
  • Fails during power outages.  Without electricity to turn on the electric fan, we had to burn huge fires in our furnace to keep the trailer at all habitable during last year's 10 day power outage.
  • Wood smokeIs meant to heat the whole house rather than a small section.  You have to burn a lot more wood in general to keep your house warm using an exterior furnace since you depend on fans to move the hot air throughout the house.  With an indoor wood stove, you can situate the stove in the population center of the house and burn small fires to heat just that area.

All told, our exterior wood furnace has definitely been better than heating with electric space heaters, but we run through wood like nobody's business.  This week, I'm going to post about some of the options we're considering --- I suspect you'll all have good advice, so please chime in!

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This post is part of our Wood Stove lunchtime series.  Read all of the entries:



Trailersteading

Edited to add:


Learn how to safely install an energy-efficient wood stove in a moibile home in
Trailersteading.  Now available for $1.99 on Amazon.




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Have a look at wood gasifiers. Those can be very efficient (up to 94%) and have a very complete combustion due to the high temperatures, so no smoke. I looked at them when my old gas furnace was acting up, but I don't have a ready source of wood available. Most gasifiers do need electricity for the fans and the control circuitry. But a solar-powered battery pack might help there in case of power outages.

Some brands I've found were atmos, [orlan (in Dutch)][] and eta. The atmos machines meet the highest European standard for pollution (EN 303-5 class 3).

I think Mark would like GEK's gasifier experiment kit. :-) Their power pallet could deliver both heat and electricity. The problem is that the need for power and electricity isn't always coupled, and I don't know how versatile this machine is. And I guess you'd also like their biochar machine. What I like is that their designs are open source, and use off-the-shelf parts where possible. If I had a house in the country with a lot of wood, I'd be tempted to get a power pallet. Or use the gasifier to make fuel for a small gas turbine driving a generator. :-)

Comment by Roland_Smith Mon Nov 8 14:55:03 2010
Those sound interesting, but way too big for us! As you'll see in a later post, I'm coming to realize that as tiny as our trailer is, we need a very, very small option.
Comment by anna Mon Nov 8 17:54:38 2010

It does not only depend on size, but also on how well insulated your trailer is, how warm you want it and if you also want it to heat tapwater. The gas heater for my flat is rated between 6-24 kW.

You can also get a modern wood stove. Those can be up to 80% efficient. But do get a model that takes its combustion air from the outside. Preferably by means of a concenctric pipe around the exhaust, so the exhaust gasses pre-heat the combustion air.

A soapstone stove gives off an even heat long after the wood has burned down, but can still has a hot fire for efficient combustion. They do tend to be heavy, though.

Or maybe a rocket stove mass heater will do?

Comment by Roland_Smith Mon Nov 8 18:30:26 2010
I'd be curious to hear your take on this page --- http://www.woodheat.org/outdoorair/outdoorairmyth.htm. Do you think the guy's arguments make sense?
Comment by anna Mon Nov 8 19:07:44 2010

I don't agree with his assessment. (a bit long and maybe rambling, it's late and I need to catch some sleep)

When open coal furnaces were common in the Netherlands, there were large numbers of fatalities and hospitalizations due to carbon monoxide poisoning. This decreased somewhat when most people switched to natural gas. But still there were a significant number of deaths and hospitalizations. So at a certain moment open hearts and central heating devices >20 kW were banned. Between 1970 an 2008, deaths trough accidental poisonings in the Netherlands dropped by 23% (source: www.swov.nl, unfortunately CO poisonings aren't registered seperately).

Current hearts >20 kW must be enclosed and get their oxygen from outside. Modern heaters all have forced ventilation for reasons I'll explain below.

I disagree that a passive air supply is useless. If the leaks around windows and doors didn't form a passive air supply, you'd poison yourself every time you lit a fire! If his argument is that a passive air supply doesn't reliably supply combustion air, than how come the normal leak spots in a house are enough?

WRT air blowing into the chimney; normally the weatherhood is designed to prevent air blowing into the chimney. There is however a tension between efficiency of a furnace and draft! A very efficient furnace will have a relatively low exhaust temperature. But a higher exhaust temperature will draft better.

Essentially for a furnace without separate and forced ventilation, the hot exhaust gas is the "engine" that has to pull the air through the gaps around doors and windows and through other leaks into the house and then through the furnace. If the house is relatively leaky, this is quite easy. But the better a house is insulated and people have been sealing leaks, the harder it becomes for the fire to get enough oxygen. Personally I think it is better for a fire to have an air supply directly to the combustion chamber, preferably with a heat exchanger to let the exhaust gases heat the incoming air, and not leave that to the vagaries of chance.

Basically, there are several things you'd want from a good (wood) heater

  • high efficiency, which requires
    • complete combustion ("afterburner")
    • low flue gas temperature (you don't want your presious heat flowing out of the chimney)
  • low chance of CO poisoning (requires a closed system)
  • no indoor pollution (requires an exhaust fan, to create a negative pressure in the furnace)
Comment by Roland_Smith Tue Nov 9 18:58:57 2010
You have to realize the efficiency thats posted on EPA'S website 63% is default from the EPA. The Caddy made by PSG is higher than 63%. Yes a good woodfurnace is expensive, but many improvements have been made to increase efficiency. We heat a 2400 sq ft victorian home with our Caddy woodfurnace. No smoke enters the home, the burn times are long and the furnace uses little wood over the old woodfurnace that was here prior. What you have and are describing in the article is an indoor wood furnace, thats probably in the 40% efficiency range. A smoke dragon to todays standard. I use less wood than my neighbors heating with wood stoves, and there homes are smaller.
Comment by Matt Sun Feb 19 21:22:43 2012

Matt --- Excellent point on the efficiency numbers. I didn't realize that until I started researching efficient stoves in more depth. Do you know of a better source that gives actual efficiency numbers for stoves?

We decided to go with a small, indoor, efficient stove, and have been very happy with it!

Comment by anna Mon Feb 20 11:23:40 2012
You could slap a couple of thermocouplers on the side of your wood furnace and power the fan off them. Free energy. You have your hot side and cold side from the furnace and cold air from winter... Look up TEG modules, and thermocouplers and read up how they work. You'll see that doesn't matter if power goes off, your fan can run just from the heat of the wood you have in your furnace. :)
Comment by David Tue Jul 24 14:58:32 2012
David --- Good idea, and timely since we were just talking about thermoelectric devices this week. We've upgraded to a different stove, but our furnace's new owners might be interested. :-)
Comment by anna Tue Jul 24 15:34:33 2012